The entrance exam is based on 3 subjects:
Biology
Chemistry
Physics/Math
You can check the course syllabi for each individual subjects on the Charles First Faculty Website (Univerzita Karlova - UK)
You are allowed to take a calculator with you for the physics/math part.
There are 30 questions in each section (total = 3 sections).
The questions are all multiple choice...and when I say multiple choice, I mean it!!! (each question can have 1 or 2 or 3 or all answers correct). So it can be quite tricky (especially for biology and chemistry).
The interview is merely a formality. Once you make it through the entrance, you're pretty much guaranteed an admission. That being said, don't underestimate the interview either. If at all you do have to attend an interview (as it's not very common), they ask you why do you want to pursue medicine and stuff like that. But for some students (in case their entrance exam scores were border-line, they might ask a question or two).
Trust me...you'll do fine and you'll get in. The biggest advantage in your case is that you're taking it early (which has some good advantages now and for the rest of your medical school career). Just revise all the basic info. If you don't mind me asking, are you sitting the exam in Prague or in any other country?